erofs: fix deadlock when shrink erofs slab

We observed the following deadlock in the stress test under low
memory scenario:

Thread A                               Thread B
- erofs_shrink_scan
 - erofs_try_to_release_workgroup
  - erofs_workgroup_try_to_freeze -- A
                                       - z_erofs_do_read_page
                                        - z_erofs_collection_begin
                                         - z_erofs_register_collection
                                          - erofs_insert_workgroup
                                           - xa_lock(&sbi->managed_pslots) -- B
                                           - erofs_workgroup_get
                                            - erofs_wait_on_workgroup_freezed -- A
  - xa_erase
   - xa_lock(&sbi->managed_pslots) -- B

To fix this, it needs to hold xa_lock before freezing the workgroup
since xarray will be touched then. So let's hold the lock before
accessing each workgroup, just like what we did with the radix tree
before.

[ Gao Xiang: Jianhua Hao also reports this issue at
  https://lore.kernel.org/r/[email protected] ]

Link: https://lore.kernel.org/r/[email protected]
Fixes: 64094a04414f ("erofs: convert workstn to XArray")
Reviewed-by: Chao Yu <[email protected]>
Reviewed-by: Gao Xiang <[email protected]>
Signed-off-by: Huang Jianan <[email protected]>
Reported-by: Jianhua Hao <[email protected]>
Signed-off-by: Gao Xiang <[email protected]>

0 files changed, 0 insertions, 0 deletions